1. Field of the Invention
The present invention relates to an antenna apparatus preferably for use in short-range communication and a communication terminal including the antenna apparatus.
2. Description of the Related Art
Radio frequency identification (RFID) systems are increasingly becoming popular as product management systems and billing and toll collection management systems. In such an RFID system, a reader/writer and an RFID tag wirelessly communicate with each other to exchange information. Each of the reader/writer and the RFID tag includes an RFID IC chip for processing a signal and an antenna for transmitting and receiving a radio signal. Predetermined information is transmitted between the antennas of the reader/writer and the RFID tag via a magnetic field or an electromagnetic field.
For example, FeliCa (registered trademark) that applies an RFID system to information communication terminals such as mobile telephones has been recently used. In Felica, a terminal itself is sometimes used as a reader/writer or an RFID tag. On the other hand, since communication terminals decrease in size and increase in functionality, there is not sufficient space for an antenna in the casings of the communication terminals. In order to solve this problem, for example, a configuration disclosed in WO2010/122685/A1 is sometimes used. In this configuration, a small coil conductor is connected to an RFID IC chip and a radio signal is transmitted from a conductive layer that is adjacent to the coil conductor and has a large area. The conductive layer functions as a radiation element (booster antenna) and is magnetically coupled to the coil conductor via an opening of the conductive layer. With this configuration, since a thin metal film can be used as the conductive layer, the conductive layer can be formed in narrow space between a printed circuit board and a terminal casing.
As the conductive layer (booster antenna), a metal film may be prepared as described above. Alternatively, in a case where the terminal casing is a metal casing, the metal casing itself may be used as the booster antenna. In this case, it is desired that the metal casing be connected to the ground of a circuit in the terminal casing. More specifically, it is desired that the metal casing be connected to the ground of a printed circuit board in the terminal casing. In the terminal casing, for example, a power supply circuit and a high-frequency signal processing circuit are formed. By using the metal casing as the ground, a ground potential in the terminal casing can become more stable. As a result, the operations of various circuits can become more stable.
However, in a case where the ground of the printed circuit board and the metal casing are connected, an antenna characteristic may be deteriorated in accordance with a connection method.